Why Hot Funds
Are Tripping Up
Some Investors

ETFs, Which Are Meant
To Track Benchmarks,
Increasingly Go Astray

Fresh troubles are emerging in a number of exchange-traded funds, shedding light on the ways the increasingly popular investment tools can trip up investors.

ETFs, which are essentially mutual funds that trade on exchanges like shares of stock, were designed to match the performances of various market benchmarks, such as the Standard & Poor's 500-stock index. They still occupy a relatively small corner of the investment universe, but they have blossomed in the past few years, thanks to their low fees and tax advantages,...